As one of the routine tasks in weather observation various kinds of weather parameters such as the wind direction, the wind velocity, the temperature, the humdity, the atmospheric pressure and so on are measured by an observation instrument provided in a balloon, whereby the measurement values are sent to the receiving station on the ground by means of radiowave radio transmission. Such observation instrument as is provided in a balloon is called a radiosonde.
It is necessary for a radiosonde to operate from the time the radiosonde is freed on the ground till it flies up to the altitude of about 30 km, whereby as the ambient conditions, temperature ranges from +40.degree. C.--80.degree. C., atmospheric pressure from 1040 mb-1 mb and humdity from 1%-100%, prevail, constituting very severe conditions for the observation instrument. Further the radiosonde must be constructed rigidly against the vibration and shacks sustained during flying. Further it is expendible, because it cannot be recovered later.
Under such circumstances it is necessary that the instruments for a radiosonde should be compact, light and cheap and further should maintain stable characteristics even under the above mentioned severe ambient conditions. On the other hand the time during which the radiosonde should maintain the characteristics in one observation is several hours at most.
Consequently it must be considered that the measuring circuit a for radiosonde should meet requirements completely different from those for the stationary installations on the ground or for ordinary equipment in ordinary daily life. As to the conventional method for measuring various kinds of weather elements by means of a radiosonde, a measuring element such as thermister or carbon hygrometer whose electrical resistance changes in accordance with the change of the state of the physical parameter, for example, in the case of the weather situation, is used as the weather sensing element, by means of the change of whose electrical resistance the change of the state of various weather factors is detected. In consequence, the above mentioned weather sensing element is connected to a circuit such as a blocking oscillator or a multivibrator whose oscillation frequency is determined by the resistance or the capacitance in the circuit in such a manner that the change of the weather conditions is converted into a change of the oscillation frequency. Such combination of the oscillation circuit with the weather sensing element is an example of the resistance-frequency converting circuit.
Hereby the blocking oscillator needs an oscillation transformer, so that both the weight and the size of the circuit becomes large and further the oscillator itself becomes expensive. Namely the blocking oscillator is not suited as the device for a radiosonde to be mounted in a balloon as an expendible element.
Under such circumstances until now instead of a blocking oscillator a resistance-frequency converting circuit consisting of a multivibrator without a transformer has been proposed for a radiosonde.
FIG. 1 shows the circuit diagram of the conventional resistance-frequency converting circuit for a radiosonde in which a multivibrator is used. FIG. 2 shows wave forms at the points shown in FIG. 1.
In FIG. 1, 1 and 2 are the inverters for NOT-circuit, 3 the resistance whose value is R.sub.S, 4 the resistor whose resistance is R, 5 the weather sensing element whose resistance value is variable (the variable resistance value is let R.sub.X), 6 the condenser whose capacitance is C, 7 the output terminal, V.sub.DD the voltage at the positive terminal of the power source and V.sub.SS the voltage at the negative terminal of the power source.
In the inverters 1 and 2 the complementary symmetry metal-oxide semiconductor integrated circuits (hereinafter called C MOS IC) are used, because C MOS IC possesses such advantages as low electric power consumption, high noise immunity, wide operation voltage range, wide operation temperature range, high input impedance and so on.
Generally C MOS IC includes a protective circuit in the form of a diode connected in such a manner that when a voltage higher than the above mentioned V.sub.DD or lower than the above mentioned V.sub.SS is applied to the input terminal of the C MOS IC, the part above V.sub.DD or below V.sub.SS can be clipped.
In consequence the wave at the point P3 in FIG. 1 assumes, as is shown in P3 in FIG. 2, a form in which it is clipped during the intervals t.sub.1 and t.sub.2 during which the applied voltage is above V.sub.DD or below V.sub.SS. The wave forms in FIG. 2 respectively correspond with those appearing at the points with the corresponding indication figures in FIG. 1.
The input terminal of the inverter 1 is connected to P2, the connecting point of the weather sensing element 5 and the condenser 6 through the resistor 3. If the voltage at the point P2 is higher than the threshold level V.sub.1 of the inverter 1, the level of the output of the inverter 1 becomes low. When this low level output of the inverter 1 is put in the input terminal of the inverter 2 the level of the output of the inverter 2 becomes high.
Because now the level at the output terminal of the inverter 2, namely at the point P1 is high while the level at the output terminal of the inverter 1, namely at the point P4 is low, a current runs from the point P1 to the point P4 through the condenser 6, the weather sensing element 5 and the resistance 4 in such a manner that, as is shown in t.sub.3 of P2 in FIG. 2, the voltage at the point P2 is gradually reduced in accordance with the time constant of the circuit consisting of the condenser 6, the weather sensing element 5 and the resistor 4. When thus the voltage at the point P2 has reached the threshold level V.sub.1 of the inverter 1, the output of the inverter 1 changes from the low level to the high level while the output of the inverter 2 changes from the high level to the low level. Consequently at this time a current runs from the point P4 to the point P1 through the resistor 4, the weather sensing element 5 and the condenser 6, whereby, as is shown in t.sub.4 of P2 in FIG. 2, the voltage at the point P2 increases gradually in accordance with the time constant of the circuit consisting of the resistor 4, the weather sensing element 5 and the condensor 6 until it reaches the threshold level V.sub.1 of the inverter 1, when the output of the inverter 1 and that of the inverter 2 are respectively inverted. After then the above mentioned operation is repeated so as to maintain the oscillation.
Let us suppose that no current flows through the resistance 3, then in FIG. 2 ##EQU1## Hereby V.sub.SS = 0. As to the oscillation frequency f: ##EQU2## Hereby K is a constant. In practice a current flows through the resistor 3 and it is an object of the present invention to solve the phenomenon due to this current.
It can be understood from (1) that the oscillation frequency is reciprocally proportional to the resistance value of the weather sensing element.
On the other hand at the point in time at which the output of the inverter 2 is inverted the voltage at the point P2 becomes equal to the power source voltage applied to the inverters 1 and 2. Namely, at this time at the point P2 in FIG. 2 EQU v2 - v1 = v.sub.dd - v.sub.ss
whereby the peak value of the voltage at the point P2 is higher than the threshold level V.sub.1 of the inverter 1 by the power source voltage (V.sub.DD - V.sub.SS), so that as to t.sub.3 in FIG. 2 a voltage higher than the voltage V.sub.DD at the positive terminal of the power source is applied to the input terminal of the inverter 1 during the time interval t.sub.1.
When, as explained before, the voltage applied to the input terminal of C MOS IC exceeds the power source voltage V.sub.DD, the excessive part is clipped by means of the protection means as is indicated by t.sub.1 of P3 in FIG. 2 so that a current flows into the input terminal of the inverter 1 through the resistor 3 during the time interval t.sub.1.
The current flowing into the point 2 through the resistor 4 and the weather sensing element 5 flows to the resistor 3 and the condenser 6. Namely during the time interval t.sub.1 the circuit is equivalent to one in which the series circuit of the resistor 4 and the weather sensing element 5 is connected in parallel with the resistor 3. Namely in FIG. 2, the time constant during t.sub.1 differs from that during (t.sub.3 - t.sub.1) in such a manner that the time constant for determining the oscillation frequency alters, so that the equation (1), is not fulfilled in reality, whereby the resistance value R.sub.X of the weather sensing element 5 is not precisely in reciprocal proportion to the oscillation frequency.
A similar phenomenon takes place during t.sub.2.
Thus, it is impossible to obtain the change of the resistance value of the weather sensing element 5 precisely from the frequency of the multivibrator in accordance with the equation (1).
It is an object of the present invention to solve the problem present in conventional circuits, wherein the change of the resistance value of the weather sensing element cannot be obtained precisely.
An object of the present invention is to provide a multivibrator by means of which the change of the resistance value of the weather sensing element can be precisely converted into a frequency signal.
Another object of the present invention is to provide a multivibrator by means of which the change of the resistance value of the weather sensing element can be precisely converted into a frequency signal in accordance with a certain determined formula.
Further another object of the present invention is to provide a resistance-frequency converting circuit for weather observation by means of which the change of the resistance value of the weather sensing element can be precisely converted into a frequency signal.
Further another object of the present invention is to provide a compact and light resistance-frequency converting circuit for weather observation.
Further another object of the present invention is to provide an inexpensive resistance-frequency converting circuit for weather observation that is insensitive to a wide range of severe ambient conditions, consisting of simple, inexpensive and easily obtained parts.
Further, another object of the present invention is to provide resistance-frequency converting circuit for weather observation to be considered as expendable.